DESCRIPTION (Adapted from the Applicant's Description): Unconjugated hyperbilirubinemia is the most common clinical condition in the newborn period. Although generally a benign developmental phenomenon, hyper-bilirubinemia can become severe and result in neurologic injury with profound long term adverse neurodevelopmental sequelae as evidenced by the reappearance of kernicterus in near term infants subject to early hospital discharge. The pathogenesis of hyperbilirubinemic encephalopathy remains unclear but central to its development is the passage of bilirubin across the blood-brain barrier (BBB) into the central nervous system (CNS). Recently, an ATP dependent integral plasma membrane transporter, phosphoglycoprotein (P-gp) has been identified. P-gp is expressed in abundance on the luminal aspect of brain capillary endothelial cells and limits the brain influx of a wide variety of unrelated lipophilic compounds. In preliminary studies using an unique P-gp deficient null mutant transgenic mouse line, the investigators observed that in the absence of brain capillary endothelial cell P-gp expression brain bilirubin influx is significantly increased. This finding implies that P-gp plays an important role in preventing the influx of bilirubin into the CNS, a concept of direct relevance to the pathogenesis of neonatal hyperbilirubinemic encephalopathy. The proposed experiments are designed to test the following two hypotheses: (1) that bilirubin is a substrate for P-gp and (2) that both the bilirubin-P-gp interaction and brain P-gp expression are developmentally regulated. The investigators propose to determine the substrate specificity of bilirubin for P-gp using (i) radioligand binding studies of bilirubin in brain tissue from control and P-gp deficient mice at different developmental ages; and (ii) in vitro studies of bilirubin uptake and efflux by wild type mouse brain capillary endothelial cells in culture subject to modulation by a P-gp inhibitor, metabolic inhibition, and an anti-P-gp monoclonal antibody. These studies will be complemented by in vivo investigations of brain bilirubin influx in control and P-gp deficient null mutant mice in the presence or absence of pretreatment with a P-gp inhibitor. The developmental expression of P-gp in mouse brain will be determined using semi-quantitative RT-PCR and Western imununoblots. Results of the proposed studies are expected to test above hypotheses. The information obtained may provide novel insights regarding the role P-gp plays in attenuating brain bilirubin content and serve as an impetus towards developing modalities that increase BBB P-gp expression in newborns thereby enhancing protection against neonatal bilirubin neurotoxicity.